Evans guided Quins to the Aviva Premiership title last term and the Amlin Challenge Cup in 2011 and has been a huge success story in South West London since joining from Auckland five years ago.
Widely regarded as one of the world's most-prominent playmakers, Evans' capture is a huge boost for the Baa-Baas as they look to provide the fierce fixture that the Lions are looking for prior to the three-match series with the Wallabies.
Evans is joined by Italian hooker Leonardo Ghiraldini and former England centre Mike Tindall, with Tindall joining the coaching staff alongside triple Lions tourist Dai Young.
The trio will link up with France back rower Imanol Harinordoquy, Italy skipper Sergio Parisse and star prop Martin Castrogiovanni, Ospreys' Samoa scrum-half Kahn Fotuali'I and Springbok centre Jaque Fourie, the man who scored the crucial late try that cost the last lost of Lions a chance of series success in South Africa.
"This year's summer tour is a very important one for our club," said Barbarians tour manager Derek Quinell, himself a series winning Lion in 1971 and a tourist twice more in '77 and '80.
"It is a great honour to play the Lions as well as England and we are very conscious of the need to invite the strongest possible squad to do justice to two great fixtures.
"Mike (Tindall) played for us for the first time last year and made a big impression both on the field, where he started against England, Ireland and Wales, but also off the field.
"He is now doing very well as a player/coach with Gloucester and we have asked him to fulfil a similar role with the Barbarians in May and June.
"As the weeks go by and national squads are selected and domestic cup and league competitions reach their final stages, we are monitoring who will be available to us.
"We then decide on the best balance of our squad in all positions and will make further announcements in due course."